1. Field of the Invention
The present invention relates to a method of manufacturing a photomask. More particularly, it relates to photomask blanks used for manufacturing a phase shift mask used for the manufacture of a semiconductor device.
2. Description of Related Art
In general, when a semiconductor device such as a large-scale integrated circuit (LSI) is manufactured, a photomask having a predetermined pattern thereon is fabricated from photomask blanks, and then a wafer is microfabricated through use of the photomask. The above photomask blanks consists of a quartz substrate, for instance, and photomask material (photomask film).
When the photomask is manufactured from the photomask blanks, a photosensitive resist, for instance, is applied over the photomask material, and then the photosensitive resist is exposed to form a desired pattern therein. Subsequently, the photomask (material) is etched through use of the photosensitive resist as a mask, and then the photosensitive resist is removed to form a desired pattern on the photomask. When microfabricating a wafer (in other words, when exposing a wafer) it is necessary to perform a defect inspection, on the photomask pattern prior to the processing.
Recently, in order to fine-pattern the wafer for manufacturing a LSI, the wavelength of exposure light has been reduced, and optical systems for exposure have become much more sophisticated. In addition, so-called RET (resolution enhancement technology) has been used. Examples of RET include phase shift lithographic technologies (lithographic technology using a half tone type phase shift mask, a Levenson type phase shift mask, or a phase etch type phase shift mask), OPC (optical proximity correction) mask technology, and OAI (off-axis illumination) technology.
When using the phase shift lithographic technology among RETs, the pattern of a half tone mask (referred to as a HT mask hereinafter) is inspected for defects before fine-patterning the wafer through use of the HT mask as a photomask. At this time, the wavelength of inspection light used for the defect inspection of the HT mask pattern is longer than the wavelength of exposure light used for exposure in the lithographic process. For this reason, the mask material has higher transmittance at the inspection light wavelength than at the exposure light wavelength, which depends on the characteristic of the HT mask material. For instance, whereas an ArF lithographic 6% HT mask has a transmittance of 6% to the exposure light having a wavelength of 193 nm, the mask has a transmittance of 30-50% to the inspection light when the inspection light has a wavelength of 365 nm (i-line).
On the other hand, a HT mask substrate (quartz substrate) has a transmittance of about 90% to the exposure light and also to the inspection light regardless of their wavelengths. Therefore, because the ArF lithographic 6% HT mask pattern has a transmittance of 30-50% at the inspection light wavelength as mentioned above, the contrast between the HT mask pattern and the HT mask substrate at the inspection light wavelength is not sufficient when the defect inspection of the HT mask pattern is performed. In other words, because the conventional HT mask blanks has higher transmittance at the inspection light wavelength than at the exposure light wavelength, the contrast between the HT mask pattern and the HT mask substrate within the HT mask fabricated from this HT mask blanks is insufficient.
Because the conventional photomask blanks is constructed as described above, there has been a drawback that sufficient inspection sensitivity cannot be obtained when performing the defect inspection of the HT mask pattern because of the insufficient contrast between the HT mask pattern and the HT mask substrate at the inspection light wavelength. In extreme cases, the defect inspection of the HT mask pattern can be impossible.
Additionally, with an increase in the degree of microfabrication of the pattern, the reduction of the wavelength of exposure light in order to enhance the pattern resolution has been more required. However, for the HT mask pattern, the longer the wavelength of the light incident (the wavelength of exposure light or of inspection light) on the HT mask pattern is, the higher the transmittance thereof is. Therefore, the shorter the wavelength of the exposure light relative to the wavelength of the inspection light is (the larger the difference between the wavelength of the exposure light and the one of the inspection light is), the higher the transmittance of the inspection light is relatively. Therefore, when performing the defect inspection of the HT mask pattern, there has been a drawback that sufficient defect inspection cannot be carried out.
It is advantageous to increase the transmittance of the HT mask pattern in order to enhance the resolution of the exposure pattern (that is, to enhance the optical image contrast caused by the increase of HT phase shift effect). However, when the transmittance of the HT mask pattern is increased, sufficient defect inspection cannot be performed, in the defect inspection of the HT mask pattern, similarly to the above reason.
As described above, with the conventional photomask blanks, whereas the resolution is improved, there has been a drawback that the impossibility of performing the sufficient defect inspection results in causing the defects in the pattern on the wafer due to the pattern defects on the photomask blanks. That is, there has been a drawback that the defects caused in the exposure pattern formed on the wafer result in the deterioration of quality of the wafer in itself.